Swimming pool type reactor rod control means



Feb. 25, 1964 w. D. WHITEHEAD 3,122,683

SWIMMING POOL TYPE REACTOR ROD CONTROL MEANS I Filed Oct. 30, 195 9 2 Sheets-Sheet 1 SENSING SENSING UNIT uNIT I I4- I PERIOD PERIOD SAFETY SAFETY AMPLIFIER AMPLIFIER @MAGNETGZ} ROD No'l I5 IH CURRENT ELEOTROMAONET lo AMPLIFIER LEVEL SENSING SAFETY AMPLIFIER @MAGNETQZ ROD [6 CURRENT ELEOTR MAONET I AMPLIFIER LEvEL SENSING SAFETY 9x AMP I I R L F E O U R R E ATT :|?EDCT|;(()53MAGN ET AMPLIFIER E1513 POWER LEvEL O 5|OKW I OKw L. 2 LU E 50 O OI L11 2 2 25 I D O [I i- U LLI J L O I v l l l I I I I I I l 74 75 76 77 78 79 so BI 82 83 e4 e5 uNSAFE NORMAL UNSAFE ZONE SAFE ZONE ZONE SIGMA BUS VOLTAGE INVENTOR WILLIAM D. WHITEHEAD ATTORNEY Feb. 25, 1964 w wHlTEHEAD I 3,122,683

SWIMMING POOL TYPE REACTOR ROD CONTROL MEANS 2 Sheets-Sheet 2 Filed Oct. 30, 1959 INVENTOR WILLIAM D. WHITEHEAD Om mm ATTORNEY United States Patent Ofitice 3,122,683 Patented Feb. 25, 1964 3,122,683 SWIMMLNG PGQL TYPE REACTOR ROD CQNTRUL MEANS Wiiiiam D. Whitehead, 3M2 30th St. SE., Washington, DJC. Filed (Pet. 30, 1959, Ser. No. 850,002 4 Claims. (till. 3ll7148.5) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates in general to safety systems for use in swimming pool type reactors and in particular to electronic control devices employed in such systems.

It is common practice in swimming pool reactor installations to employ an electromagnetic holding system to hold the rod assembly above the reactor core. Such a holding system provides the operator with both auto matic and non-automatic means for the immediate release of the rod which, when freed, falls by gravity into the reactor core. The act of releasing the rod to shut down the reactor is referred to as a scram.

In a typical swimming pool reactor safety system, a scram action is initiated by either a primary or a secondary scram signal. Usually a primary scram signal indicates the power level of the reactor has exceeded a predetermined limit or the period of the reactor flux or power becomes less than a preselected time interval. Likewise, a secondary scram signal indicates (1) an overradiation level due to gamma flux emerging from the pool or (2) control equipment power failure, or (3) one of the manually operated scram buttons has been actuated. Generally the length of time required to initiate a primary scram is on the order of 30 milliseconds and that of a secondary scram is on the order of 300 milliseconds.

For obvious reasons the safety system usually is designed so that a failure in the safety system itself will automatically scram the reactor, that is the safety system is designed to fail safe.

While it is essential that the safety system respond to each and every warning signal as designed, it is also important that uninterrupted reactor operation be maintained in the absence of a safety hazard.

Prior art magnet amplifiers have functioned well from the safety aspect but have been subject to frequent com ponent failure which has necessitated unnecessary shutdown of the reactor. To reduce the possibility of shutdown due to component failure, it has been common practice to employ standby equipment and an electronic monitor circuit which serves to switch out the defective magnet amplifier and to substitute the auxiliary magnet amplifier.

Prior art rod holding systems have been plagued by vacuum tube breakdown in the magnet amplifier and by insulation breakdown in the magnet, and considerable maintenance time in testing and replacement has been required. In addition, the prior art amplifiers have a gradual rather than a sharp cut-off output current curve and the adjustment for normal holding current operation has required constant attention by highly skilled operators.

Accordingly, it is an object of this invention to provide an improved electromagnetic holding control system which is adaptable to existing systems.

It is another object of this invention to provide such a control system which is simple to adjust and requires less technical skill of the operator.

It is still another object of this invention to provide such a control system which is highly reliable.

It is also an object of this invention to provide a control system which greatly reduces equipment and space requirements.

It is a further object of this invention to provide a magnet control system which minimizes voltage breakdown problems in the submerged electromagnet.

It is another object of this invention to provide a magnet control system which requires a minimum of the operators time and attention.

Other objects of this invention will be appreciated upon a more comprehensive understanding of the invention for which reference is had to the accompanying description and illustration of the invention.

In the drawings:

FIG. 1 depicts in block diagram form a rod holding system which incorporates the device of this invention.

FIG. 2 is a schematic showing of one embodiment of the magnet amplifier of this invention.

FIG. 3 is a graphical showing of the magnet current output of the embodiment shown in FIG. 2.

Briefly, the present invention provides a transistorized magnet amplifier circuit which not only affords a miniaturization of the equipment but also produces a substantially more reliable device. The device of this invention has an improved output current curve especially suited for magnet control purposes which minimizes and substantially eliminates a sentinel requirement of prior art mag net amplifiers.

A basic safety system of the type commonly employed in swimming pool type reactors is shown in FIG. 1. In this type of safety system, the rod holding assembly, not shown in any detail, includes an electromagnet which when energized mechanically connects the rod support to the drive assembly whereby operation of the drive assembly serves to position the rod at a selected point relative to the reactor core. The electromagnet associated with each rod holding assembly is individually energized by its respective magnet current amplifier and the magnitude of the output of each magnet current amplifier is determined by the input signal applied thereto via the Sigma bus. Sigma bus It) serves to sum the outputs of the level safety amplifiers H and 12 and of the period safety amplifiers 13 and 14 which are connected to the sensing units 15 and 16 and to the sensing units 1'7 and 18, respectively. In accordance with the present invention the magnet current amplifiers 19 each provide means for controlling the magnitude of the output current which effectively serves as the on-off control for each rod holding assembly. In addition, each magnet current amplifier provides means for controlling the minimum signal input requisite for proper energization of the electromagnet.

FIG. 2 is a schematic showing of the magnet current amplifier shown in the system of FIG. 1. In FIG. 2 the input voltage signal from the Sigma bus is applied across the input voltage divider, the serially connected resistances 21, 22, and 23. Transistors 3t) and 31, which may be Texas 2N1l9 silicon transistors, for example, are con nected in cascade to function as a preamplifier. In accordance with this mode of operation, the collector to emitter paths of the transistors 30 and 31 are connected via their respective collector load resistances 24 and 25 and their respective emitter bias resistances 26 and 27 across the voltage supply source 5d. The base of transistor 30 is directly connected to the variable tap on resistance 22 such that the bias voltage applied to the transistor 30 may be adjusted in accordance with the normal voltage on the Sigma bus. Resistance 28 serves to couple voltage variations at the collector to the base of the transistor 31 and thus to vary the bias thereon accordingly. Feedback resistance 29 is connected between the emitter of transistor 31 and the input voltage divider to provide gain stability. Feedback resistance 29 permits the replacemerit of either the transistor 30 or the transistor 31 with other transistor elements not necessarily identical with the original, i.e., with a different gain. It will be appreciated that the feedback resistance 29 reduces the gain to some extent and that this feedback path may be omitted where, for example, the replacement transistors are known to be identical with the original.

The output of the preamplifier, indicated at point B, is applied to first and second signal responsive channels. The first channel, which includes the transistors 32 and 33, is responsive to voltage decreases at point -B but is insensitive to voltage increases at this point. The second channel, which includes the transistors 34 and 35, is responsive to voltage increases at point B and is insensitive to voltage decreases at this point.

in the first signal responsive channel, a clamping diode 36 is connected between the point B and the variable contact of the potentiometer 41 of the voltage divider including resistances 4'2 and 43 via the diode load resistance, 44, as shown, to permit current flow in the direction of the point B. The voltage divider which includes the resistances 42 and 43 and the potentiometer 41 is, of course, connected across the voltage source to provide selected voltage levels as desired.

The base of transistor 32 is connected to the junction of diode 36 and the diode load resistance 44 such that the base to emitter bias is determined by the voltage at point B provided the voltage at point B does not exceed the voltage at the variable tap of potentiometer 41. The collector to emitter path of transistor 32 is connected via the collector load resistance 45 and the emitter bias resistance 46 across the voltage supply source St The base of transistor 33 is directly connected to the collector of transistor 32 such that the 'base to emitter bias is determined by the collector voltage of transistor 32. Transistor 33 is a high-current type transistor and in accordance with this invention all the magnet current flows therethrough. Transistor 33 may be, for example, a Philco 2N387 germanium transistor. It will be appreciated that the maximum intended magnet current, approximately 50 ma., is substantially below the current carrying capacity of the transistor 33. The emitter to collector path of transistor 33 is connected via the collector load impedance 47, which is the impedance of the electromagnet, and the emitter bias resistance 48 across the voltage source 553. A milliammeter 49 is shown serially connected between the electromagnet impedance 497 and the collector of transistor 33 to indicate magnet current.

In the second signal responsive channel, the emitter to collector path of transistor 34 is connected via the collector load impedance '51 and a portion of the voltage divider which includes the potentiometer 52 and resistance 53 across the voltage source 50. The base of transistor 34 is connected via the isolating resistor 54 to the point B such that the base to emitter bias is determined by the voltage at point B with reference to the voltage at point C, the variable tap of potentiometer 52 at point C.

Transistor 35, like transistor 33, is a high-current transistor. The emitter to collector path of transistor 35 is connected across the voltage source via the emitter bias resistance 48 such that the emitter to collector paths of transistors 33 and 35 are connected in parallel and the emitter bias resistance 48 is common to both transistor current paths.

The collector load resistance 51 of transistor 34 and the resistances 55 and 56 are serially connected across the voltage source to form a current divider. The base of transistor 35 is connected to the mid point of resistances 55 and '56 such that the base to emitter bias is dependent upon the voltage at this mid point which is in turn determined by the magnitude of the transistor 34- current through the resistance 51.

In operation, the signals from all safety circuits and devices are paralleled in Sigma bus 1-9 which acts as an auctioneering circuit with a positive DC. voltage output. This positive DC. voltage is applied to the input terminal A causing current to flow in the voltage divider, the potentiometer 22. and the resistances 21 and 23. The variable tap on potentiometer 2 2 is adjusted to allow a portion of this current to flow into the base of transistor 3%) and through the bias resistance 26 such that transistor 3'9 collector current will be biased midway on its load line as established by the collector load resistance 24, the bias impedance 25 and the voltage supply source 5t).

The transistor 31 is biased on in the same way by means of the collector to base connection between the transistors 3t) and 31 such that the transistor 31 is also normally biased midway on its load line. The resulting voltage at the point B will then rise and fall when a signal causes the input DC. voltage at point A to rise or fall. Employing Texas 2N1l9 silicon transistors, the gain of this section is about 400 with the input base current (transistor 30) set about 50 microamps.

With the point B approximately midway between its maximum and minimum voltage swing, as established by the quiescent DC. voltage at the input, point A, and the setting of the potentiometer 22, the magnet current (collector current of transistor 33) is adjusted by means of the potentiometer 41 as required, for example, to produce a 50 ma. magnet current.

As discussed above, the voltage of point A is fed, via the transistor 39 and 31 circuitry, to point B and then to the first and second signal responsive channels. Thus, at point A a voltage variation in the negative direction is coupled to the base of transistor 32 which reduces the bias voltage and decreases the current through transistor 32 to increase the voltage at the collector of transistor 32 and at the base of transistor 33. The increase of voltage at the base of transistor 33 reduces the bias thereon and this in turn reduces magnet current. When the input voltage at point A drops a sufficient amount, as in the case of an abnormally low signal or a short at the input, the magnet current is effectively cut off and the rod held by the electromagnet is released to fall into the reactor core, It will be appreciated that the voltage variation in the negative direction is also coupled to the base of transistor 34 in the second signal response channel, but that the fixed minimum bias provided by the voltage divider, the potentiometer '52 and resistance 53 prevents the bias voltage from following the input signal.

Likewise, a voltage variation in a positive direction is blocked from the transistor 32 in the first signal responsive channel by the diode 3'6, but does allect the bias on the transistor The increase in input voltage is amplified by the transistor 34, which is normally conducting, and coupled to the transistor 35 via the current divider the resistances 5-1, 55 and 56 to drive the transistor 35, which is normally nonconducting, into heavy conduction. When this occurs, the emitter of transistor 35 drops due to the IR drop across the resistance 48 and with the transistor 33 emitter and the transistor 35' emitter common, the resultant voltage drop will bias the transistor 33 off and this, in turn, will cut on" the magnet current.

FIG. 3 is a graphical presentation of the response characteristic of the magnet current amplifier of this invention. In FIG. 3, the ordinate represents magnet current measured in milliamperes and the abscissa represents input signal variations about the normal Sigma bus voltage measured in volts. The horizontal line C indicates the normal magnet current is approximately 59 ma. in this particular case. The horizontal line D indicates the current level at which the gravitational force acting on the rod is equal to the holding force of the electromagnet. Of course, below the current level of line D, in this case 25 ma, the rods will drop.

As indicated in H6. 3, the device or" this invention has a current characteristic particularly suited for magnet control applications. For example, there is a relatively wide plateau along the line C which permits slight inconsequential voltage variations without significant change in magnet current. Also, the slope of the curve to either side of the plateau is substantially similar and relatively steep which afiords a well defined scram point and a fast scram response to a danger indication.

The characteristic curve of the magnet current amplifier of this invention is in sharp contrast to the characteristic curve of prior art magnet current amplifiers which employ vacuum tubes. In particular, the curve of such prior art devices had substantially no plateau area and the cutoff curve for excess power level (voltage variation in opposite directions) was approximately /2 as steep as the comparative portion of the curve as shown in FIG. 3.

It will be appreciated that many variations of the circuitry as shown in the exemplary embodiment of FIG. 2 are possible and are within the purview of this invention. For example, temperature compensation of the device to allow for ambient temperature variations might be accomplished by the use of one or more temperature compensation elements. In particular, the resistances 26 and 42 might be of the temperature compensation variety.

in a typical embodiment of the type shown in FIG. 2, the following resistance values might be employed for the numbered impedance elements:

Moreover, it is understood that impedance elements other than the resistive impedance elements listed above might be substituted in many instances without altering the basic operation of the device. As an example, a voltage divider made up of inductive elements might be employed if desired. Finally it is understood that this invention is limited only by the scope of the claims appended thereto.

What is claimed is:

1. An electromagnet control means adapted to maintain the strength of the magnetic field of an electromagnet above a selected level when the input voltage is within a selected voltage region and to reduce the strength of said magnetic field below said selected level when the input voltage is without said selected region comprising an electrical energy source; current limiting means; means serially connecting said current limiting means and said electromagnet across said electrical energy source; said current limiting means being normally conducting and adapted to control the magnitude of current flow therethrough, said current limiting means including an input impedance responsive to voltage deviation thereacross to vary said magnitude of current flow in proportion thereto; first and second signal responsive channel means; signal applying means having a normal voltage output representative of the normal magnitude of the input signal information, said normal magnitude being within said selected voltage region; means connecting the output of said signal applying means to the inputs of said first and second channel means; means connecting said first channel means to one end of said input impedance of said current limiting means and means connecting said second channel means to the other end of said input impedance of said current limiting means such that the voltage diiference across said impedance is determined by said first and second channel means; said first and second channel means each adapted to respond to input signal variations of respective opposite direction such that said first channel means is responsive to input voltage variations in the portion of said selected voltage region above said normal voltage output of said voltage applying means and said second channel means is responsive to input voltage variations in the portion of said selected region below said normal voltage output of said signal applying means.

2. The device as claimed in claim 1 wherein said current limiting means is a high current type transistor and said input impedance is the base to emitter impedance of said high current type transistor.

3. The device as claimed in claim 2 wherein said means serially connecting said current limiting means and said electromagnet across said electrical energy source includes a series impedance connected to the emitter of said high current type transistor and means for connecting the collector thereof to said electromagnet, and the output of said second channel means is connected across said series impedance to vary the voltage drop thereacross.

4. The device as claimed in claim 3 wherein said signal applying means comprises a two transistor, cascade connected preamplifier.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTROMAGNET CONTROL MEANS ADAPTED TO MAINTAIN THE STRENGTH OF THE MAGNETIC FIELD OF AN ELECTROMAGNET ABOVE A SELECTED LEVEL WHEN THE INPUT VOLTAGE IS WITHIN A SELECTED VOLTAGE REGION AND TO REDUCE THE STRENGTH OF SAID MAGNETIC FIELD BELOW SAID SELECTED LEVEL WHEN THE INPUT VOLTAGE IS WITHOUT SAID SELECTED REGION COMPRISING AN ELECTRICAL ENERGY SOURCE; CURRENT LIMITING MEANS; MEANS SERIALLY CONNECTING SAID CURRENT LIMITING MEANS AND SAID ELECTROMAGNET ACROSS SAID ELECTRICAL ENERGY SOURCE; SAID CURRENT LIMITING MEANS BEING NORMALLY CONDUCTING AND ADAPTED TO CONTROL THE MAGNITUDE OF CURRENT FLOW THERETHROUGH, SAID CURRENT LIMITING MEANS INCLUDING AN INPUT IMPEDANCE RESPONSIVE TO VOLTAGE DEVIATION THEREACROSS TO VARY SAID MAGNITUDE OF CURRENT FLOW IN PROPORTION THERETO; FIRST AND SECOND SIGNAL RESPONSIVE CHANNEL MEANS; SIGNAL APPLYING MEANS HAVING A NORMAL VOLTAGE OUTPUT REPRESENTATIVE OF THE NORMAL MAGNITUDE OF THE INPUT SIGNAL INFORMATION, SAID NORMAL MAGNITUDE BEING WITHIN SAID SELECTED VOLTAGE REGION; MEANS CONNECTING THE OUTPUT OF SAID SIGNAL APPLYING MEANS TO THE INPUTS OF SAID FIRST AND SECOND CHANNEL MEANS; MEANS CONNECTING SAID FIRST CHANNEL MEANS TO ONE END OF SAID INPUT IMPEDANCE OF SAID CURRENT LIMITING MEANS AND MEANS CONNECTING SAID SECOND CHANNEL MEANS TO THE OTHER END OF SAID INPUT IMPEDANCE OF SAID CURRENT LIMITING MEANS SUCH THAT THE VOLTAGE DIFFERENCE ACROSS SAID IMPEDANCE IS DETERMINED BY SAID FIRST AND SECOND CHANNEL MEANS; SAID FIRST AND SECOND CHANNEL MEANS EACH ADAPTED TO RESPOND TO INPUT SIGNAL VARIATIONS OF RESPECTIVE OPPOSITE DIRECTION SUCH THAT SAID FIRST CHANNEL MEANS IS RESPONSIVE TO INPUT VOLTAGE VARIATIONS IN THE PORTION OF SAID SELECTED VOLTAGE REGION ABOVE SAID NORMAL VOLTAGE OUTPUT OF SAID VOLTAGE APPLYING MEANS AND SAID SECOND CHANNEL MEANS IS RESPONSIVE TO INPUT VOLTAGE VARIATIONS IN THE PORTION OF SAID SELECTED REGION BELOW SAID NORMAL VOLTAGE OUTPUT OF SAID SIGNAL APPLYING MEANS. 